Most people want to have a mouth full of dazzling white teeth, as the teeth constitute an important part of the overall picture of the human face especially when smiling. In contrast hereto strongly discoloured teeth entail an unhealthy and in certain cases even repellent "look".
For many years, crowns or dentures were seen as the only means for avoiding e.g. the yellowing of teeth coming with age, discolouration of teeth due to long term intake of the antibiotic tetracycline during childhood, or the yellowish brown to black discolouration of teeth as a consequence of coffee drinking, tobacco smoking/chewing etc.
Even though techniques for bleaching teeth have been known for many years, tooth bleaching has never been used extensively, until recently.
The first published tooth bleaching technique used oxalic acid as the bleaching agent and was reported by Chaple in the 1877. Soon after, the first report of peroxide used as a bleaching agent was published by Harlan in 1884. Even though quite a number of bleaching agents have been suggested since then peroxide is today still the most commonly used agent.
The structure of teeth
A mammal tooth is chiefly composed of four main constituents namely the "cementum", the "dental pulp", the "dentin", and the "enamel".
The "cementum" is bonelike tissue forming the outer surface of the root of the tooth. The "dental pulp" consists of sensitive tissue containing arteries, veins, and lymphatic and nerve tissue. The "dentin" which surrounds the dental pulp forms the major part of the tooth. The dentin is dense bonelike calcareous tissue. On the outside of the tooth there is a hard porous layer composed of hydroxyapatite mineral crystals having a natural opaque white or off-white colour. This outer porous layer is called the "enamel".
Tooth discolouration
Tooth discolouration can be caused by a variety of intrinsic and/or extrinsic influences. In general stains are divided into two main categories: 1) extrinsic stains and 2) intrinsic strains.
1) Extrinsic stains
Extrinsic stains are mainly caused by the daily intake of substances, such as foods and beverages through the mouth, and/or the use of tobacco products etc. These substances tend to adhere to the enamel's hydoxyapatite structure and hereby discolour the teeth and/or reduce the whiteness of the teeth.
Over a period of years extrinsic stains may penetrate the enamel layer and gradually give intrinsic discolourations.
2) Intrinsic stains
Intrinsic stains are the term used for stains which have penetrated the tooth structure (i.e. discolouration within the tooth matrix). Such stains can arise as described above or be caused by agents including haematological agents and certain drugs, or be due to dental pulp necrosis or developmental abnormalities.
For instance, degradation products from the body may cause discolorations. Excessive intake of tetracycline or fluoride during a long period of illness has been found to release degradation products into the dentinal tubules during the development of tooth enamel causing some degree of discolouration. The severity of such discolouration depends on the time and duration of intake of the medicine.
Further, dental pulp necrosis entails Haemorrhagic discolorations and is a result of blood degradation. If the pulp necrosis is caused by e.g. caries degradation of proteins the tooth/teeth will become greyish-brown.
In the case of traumatic pulp death the tooth will turn yellow-brown. It is believed that such a discolouration is caused by haemolysis of red blood cells entering the dentinal tubules.
Tooth bleaching
Before conducting tooth bleaching it is important to assess the type of stain, as different types of stains need different approaches and/or bleaching agents.
Certain extrinsic stains, which occur on the surface or subsurface of the teeth, can be removed by regular intense mechanical brushing of the teeth with cleansing agents containing abrasives and surfactants. However, not all extrinsic stains can be removed this way and require bleaching agents which inhibit non-enzymatic browning reactions.
Intrinsic stains are located in the tooth matrix and cannot be removed or prevented by intense mechanical brushing of the teeth. Removal of such discolourations requires bleaching agents capable of penetrating into the teeth structure. Hydrogen peroxide is an example of such an agent, which can be used for both extrinsic and intrinsic stains.
Hydrogen peroxide can be used for many types of stains e.g. stains residing in the dentin, such as stains caused by tetracycline.
Even though hydrochloric acid is not regarded as being a bleaching agent it is known to be capable of removing stains caused by fluorosis, as it dissolves the surface of the teeth.
However, the use of such agents can inflict severe tooth damage or at least irritation in the oral cavity. Consequently, such agents are for safety reasons not suitable for "home-use" by the private consumer and should only be used precautiously by professionals.
Bleaching techniques
Bleaching techniques are usually divided into two main categories:
a) non-vital bleaching techniques, and PA1 b) vital bleaching techniques.
a) Non-vital bleaching
The non-vital techniques give the most effective results but also have the greatest potential hazard. One non-vital bleaching technique uses sodium perborate and 35% hydrogen peroxide as the active ingredient.
b) Vital bleaching
Products sold for vital bleaching techniques can be divided into three main groups a) "in-office" bleaching products, b) dentist prescribed, home applied bleaching products, and c) over-the-counter bleaching kits.
For further information concerning the categorisation of tooth bleaching products and techniques we refer to Van B. Haywood, (1992), Periodontology and Restorative Dentistry, p. 142-149.
One of the most commonly used "in-office"-techniques combines the use 30% hydrogen peroxide with heat and light treatment to speed up the oxidation reaction (i.e. the removal of stains).
Another method, using a "dentist prescribed, home-applied"-bleaching product, involves the use of 10% urea peroxide (carbamide peroxide). The teeth are bleached in a mouth tray, containing the bleaching agent, placed upon the teeth of the patient.
Over-the-counter kits which can be used for bleaching teeth include products such as toothpastes and mouth washes having from 3% to 6% hydrogen peroxide and are sold directly to the consumer, without prescription by a dentist.
Comments to prior art
Most of the above mentioned prior art methods/techniques involve the use of peroxides and/or other agents in concentrations which are not safe for "home-use" by the private consumer due to the risk of damaging the teeth and/or the oral tissue.
Furthermore, effective concentrations of e.g. hydrogen peroxide exceed the allowed limits in certain countries.
Products comprising low concentration of bleaching agents, such as hydrogen peroxide, are considered to have slow bleaching effect.
Therefore, there is a need for providing safe tooth bleaching compositions, which do not comprise harmful concentrations of peroxide and/or other hazardous agents. It is further desirable that such tooth bleaching compositions can be used as components in conventional oral care products for "home-use" by the private consumer.